1. Field of the Invention
The present invention relates to an image recording apparatus having an image recording means for recording a two-dimensional image on a recording sheet such as a photosensitive medium held on the outer or inner circumferential surface of a drum, an a method of generating a pixel clock, which is preferably applicable to such an image recording apparatus.
2. Description of the Related Art
There have heretofore been known external surface scanning light beam image recording apparatus for recording a two-dimensional image on the entire surface of a recording medium on the outer circumferential surface of a cylindrical drum by rotating the drum, scanning the recording medium with an intensity-modulated light beam emitted from an optical system in a main scanning direction, and moving the optical system in an axial direction of the drum thereby to scan the recording medium in an auxiliary scanning direction transverse to the main scanning direction. See, for example, Japanese laid-open patent publications Nos. 5-207250, 9-149211, and 10-16290, for details.
The drum, which has a diameter of 300 mm and a length of 1 m and is made of aluminum or the like, of those disclosed external surface scanning light beam image recording apparatus actually suffers various dimensional errors. For example, the drum has various diameter and outer circumferential surface dimension variations, which fall within a machining tolerance range, caused in the manufacturing process, and also have eccentricity errors introduced when the drums are assembled. Consequently, even when the drum is rotated at a constant speed, the circumferential speed of the outer circumferential surface of the drum is not constant. With the irregular circumferential speed, when an image is plotted on the photosensitive medium by the light beam that is intensity-modulated, e.g., selectively turned on and off, with pixel clock pulses at constant intervals, the recorded image tends to be unduly expanded or contracted in local regions.
One solution proposed in the known apparatus has been to measure a distortion of an image which has been plotted with pixel clock pulses and correcting the spaced intervals of the pixel clock pulses when the image is actually recorded for thereby minimizing expansions and contractions of the image. According to the system disclosed in Japanese laid-open patent publication No. 5-207250, the frequency-diving ratio of a PLL circuit which generates pixel clock pulses is varied to correct the spaced intervals of the pixel clock pulses. However, the disclosed solution is disadvantageous in that the image tends to be distorted due to a pull-in time of the PLL circuit at the time the frequency-diving ratio thereof is varied.
The technique revealed in Japanese laid-open patent publication No. 9-149211 corrects the spaced intervals of pixel clock pulses by changing an input voltage applied to a voltage-controlled oscillator. The revealed technique is also problematic in that the image is liable to suffer a new distortion owing to the temperature characteristics of the voltage-controlled oscillator.
It has been proposed to use a programmable delay line or a plurality of delay lines to correct clock pulse positions for solving the problem disclosed in Japanese patent laid-open publication No. 5-207250 or Japanese laid-open patent publication No. 9-149211. However, a correcting circuit made of inexpensive delay line or lines fails to achieve a required level of accuracy and resolution.
To eliminate the above difficulties, the system disclosed in Japanese laid-open patent publication No. 10-16290 employs, as shown in FIGS. 13 and 14 of the accompanying drawings, a rotary encoder 1 mounted on the shaft of a motor for rotating the drum to generate a fundamental clock whose frequency is multiplied to produce an original clock by a PLL circuit 2. The pulses of the original clock are digitally counted by a counter 3. The counter 3 comprises a preset down counter and functions as a frequency divider, and is also referred to as a frequency divider. Based on the count from the counter 3, a CPU 4 reads correcting data from a correcting data memory 5. Based on the read correcting data, a control circuit 6 selects a frequency-dividing ratio of the counter or frequency divider 3 to divide the frequency of the original clock from the PLL circuit 2 by 7, 8, or 9.
The disclosed system can achieve a required level of accuracy and resolution because the clock pulse positions are corrected digitally by the counter 3 and a clock adjusting means 7 which is made up of the CPU 4, the corrective data memory 5, and the control circuit 6.
The correcting data are produced as follows: The circumferential surface of the drum that corresponds to a full image surface is developed into a flat rectangular surface, which is divided along main and auxiliary scanning directions into a mesh pattern of small rectangular cells or grip points, and correcting data for the respective rectangular cells or grid points are stored as original correcting data in the correcting data memory 5. The CPU 4 calculates, from coordinates to be recorded next that are obtained by counting pixel clock pulses and the stored original correcting data, correcting data for the coordinate position to be recorded next, and determines a recording time based on the calculated correcting data.
However, the above technique is disadvantageous in that when an exposure recording condition such as a dot per inch (DPI) with respect to the photosensitive medium is changed, it is necessary to calculate and regenerate original correcting data for respective grid points of the full image surface, and hence the productivity is greatly reduced.
The foregoing drawback may be eliminated by generating original correcting data for respective grid points of the full image surface with respect to each exposure recording condition and storing the generated original correcting data in the correcting data memory. This approach is highly costly because a large-storage-capacity semiconductor memory or a hard disk is needed as the correcting data memory for storing such original correcting data.
If clock pulse positions are to be corrected in view of the expansion or contraction of the drum due to environmental temperature changes, then it is necessary to store original correcting data for each temperature, resulting in a possible further increase in the cost. The system shown in FIGS. 13 and 14 is also problematic in that it requires a complex control process for the control circuit 6 to set frequency-dividing ratios in the counter or frequency divider 3 for small variations of clock pulse positions to be corrected, the CPU 4 requires a considerable power to generate a correcting table for setting frequency-dividing ratios, and the correcting data memory 5 needs a large storage capacity for storing the calculated data.
In addition, the original clock outputted from the PLL circuit 2, whose frequency is 8 times the frequency of the pixel clock, is usually frequency-divided by 8 and partly frequency-divided by 7 or 9 by the counter or frequency divider 3, for the correction of pixel clock positions. Therefore, pixel clock positions are corrected in fixed positions along the main scanning direction at all times, so that an image produced on the photosensitive medium tends to suffer a quality degradation such as a striped irregularity or a moirxc3xa9 pattern.
It is therefore an object of the present invention to provide an image recording apparatus which has a relatively simple arrangement capable of stably correcting a distortion of an image, such as an expansion or a contraction, due to an error of a mechanical system for holding a recording sheet for recording the image thereon, for thereby accurately recording or reproducing the image on the recording sheet, and a method of generating a pixel clock in such an image recording apparatus.
Another object of the present invention is to provide an image recording apparatus which is capable of generating and holding data for correcting the level of graphical accuracy efficiently with a resource saver, against a distortion of an image, such as an expansion or a contraction, due to an error of a mechanical system for holding a recording sheet for recording an image thereon, and a method of generating a pixel clock in such an image recording apparatus.
Still another object of the present invention is to provide an image recording apparatus which will not produce a quality degradation such as a striped irregularity or a moirxc3xa9 pattern in a recorded image, and a method of generating a pixel clock in such an image recording apparatus.
According to the present invention, there is provided an image recording apparatus has image recording means for scanning a recording sheet in a main scanning direction to record an image on the recording sheet, the image recording means being movable in an auxiliary scanning direction Y substantially perpendicular to the main scanning direction X to record a two-dimensional image on the recording sheet. The image recording apparatus comprises means for detecting recording position information in the main scanning direction, original clock generating means for generating an original clock based on the recording position information in the main scanning direction, decimation counting means for counting pulses of the original clock and outputting a decimating instruction to decimate a pulse from the original clock each time a preset count is reached, decimating means for decimating a pulse from the original clock based on the decimating instruction, and frequency-dividing means for frequency-dividing a decimated clock at a fixed frequency-dividing ratio and outputting the frequency-divided clock as a pixel clock for recording the image.
With the above arrangement, since the frequency of the pixel clock is varied by decimating the original clock based on the preset count, the image can accurately be recorded on the recording sheet by determining in advance the preset count depending on the positional relationship between the recording sheet and the image recording means.
The recording sheet may comprise a photosensitive medium such as PS plate, a photosensitive film, or the like, or a printing sheet of paper, or a metal plate such as an aluminum sheet or the like.
The image recording means may comprise an optical system for emitting a light beam to be applied to the recording sheet. The optical system allows pixels having a diameter of 10 xcexcm or less to be produced with the light beam emitted thereby. If a PS plate is used as the recording sheet, it allows the image recording apparatus to be constructed as a CTP (Computer To Plate) apparatus.
The image recording means may comprise an ink jet head 134 for applying an ink I to the recording sheet, and the image recording apparatus may further comprise a rotatable drum with the recording sheet mounted on an outer circumferential surface thereof, means for controlling the ink jet head to apply the ink to scan the recording sheet on the rotatable drum in the main scanning direction to record the image on the recording sheet, and means for moving the ink jet head in the auxiliary scanning direction Y along an axis of the rotatable drum to record the two-dimensional image on the recording sheet. With this arrangement, the image recorded on the recording sheet can maintain a desired level of dimensional accuracy irrespective of variations of the diameter of the drum.
The image recording apparatus may further comprise a rotatable drum with the recording sheet mounted on an outer circumferential surface thereof, means for controlling the optical system to apply the light beam to scan the recording sheet on the rotatable drum in the main scanning direction to record the image on the recording sheet, and means for moving the optical system in the auxiliary scanning direction along an axis of the rotatable drum to record the two-dimensional image on the recording sheet. With this arrangement, the image recorded on the recording sheet can maintain a desired level of dimensional accuracy irrespective of variations of the diameter of the drum.
The image recording apparatus may further comprise a drum with the recording sheet mounted on an inner circumferential surface thereof, means for rotating the optical system about an axis of the drum to cause the light beam emitted from the optical system to scan the recording sheet on the rotatable drum in the main scanning direction to record the image on the recording sheet, and means for moving the optical system in the auxiliary scanning direction along the axis of the drum to record the two-dimensional image on the recording sheet. With this arrangement, the image recorded on the recording sheet can maintain a desired level of dimensional accuracy irrespective of variations of the diameter of the drum.
The image recording apparatus may further comprise means for detecting information per revolution of the drum, the decimation counting means comprising means for resetting the count of the original clock and thereafter starting to count the original clock to the preset count when the information per revolution of the drum is detected. Therefore, if necessary, a correcting value can be varied for each main scanning line thereby to facilitate a fine correcting process.
Similarly, the image recording apparatus may further comprise means for detecting information per revolution of the optical system, the decimation counting means comprising means for resetting the count of the original clock and thereafter starting to count the original clock to the preset count when the information per revolution of the drum is detected. Therefore, if necessary, a correcting value can be varied for each main scanning line thereby to facilitate a fine correcting process.
The image recording apparatus may further comprise random number generating means for generating a random number, the decimation counting means comprising means for setting a first preset count of the original clock after the count is reset to a value corresponding to the random number generated by the random number generating means, and outputting a decimating instruction to set a second and subsequent preset count of the original count to the preset count. Consequently, pixel clock positions are prevented from being corrected in fixed positions along the main scanning direction at all times, so that an image produced on the recording sheet does not suffer a quality degradation such as a striped irregularity or a moirxc3xa9 pattern.
The first preset count of the original clock after the count is reset may be set to the random number between a value of 0 and the preset value. The random number generating means is thus simple in structure, and corrected positions are prevented from being displaced largely.
The preset count may be determined depending on either one of a diameter of the drum, a temperature of the image recording apparatus, or a thickness of the recording sheet. Thus, images to be recorded can accurately be corrected with respect to such various parameters.
According to the present invention, there is also provided a method of generating a pixel clock to correct a graphical accuracy distortion of an image recorded on a recording sheet in an image recording apparatus which has image recording means for scanning a recording sheet mounted on a mechanical component in a main scanning direction to record an image on the recording sheet per pulse of the pixel clock, the image recording means being movable in an auxiliary scanning direction substantially perpendicular to the main scanning direction to record a two-dimensional image on the recording sheet, the method comprising the steps of generating mechanical component correcting data based on a relative positional relationship between the mechanical component and the image recording means, and controlling a time to generate the pixel clock to energize the image recording means based on the mechanical component correcting data and proportional component correcting data corresponding to recording conditions for the image recording means to record the image on the recording sheet, when the image is recorded on the recording sheet by the image recording means.
With the above arrangement, the time to generate the pixel clock is controlled based on the mechanical component correcting data based on the relative positional relationship between the mechanical component and the image recording means and the proportional component correcting data Dp corresponding to recording conditions for the image recording means to record the image on the recording sheet.
The mechanical component correcting data include main and auxiliary scanning direction components each kept for one line, and the proportional component correcting data is not kept but recalculated each time recording conditions are determined. Therefore, the amount of correcting data that is generated and held is minimized.
The proportional component correcting data comprises either one of data of the recording resolution on the recording sheet, the thickness of the recording sheet, and the temperature in the image recording apparatus, for example.
The image recording means may comprise an optical system for emitting a light beam to be applied to the recording sheet.
The image recording means may comprise an ink jet head for applying an ink to the recording sheet, the image recording apparatus further comprising a rotatable drum with the recording sheet mounted on an outer circumferential surface thereof, means for controlling the ink jet head to apply the ink to scan the recording sheet on the rotatable drum in the main scanning direction to record the image on the recording sheet, and means for moving the ink jet head in the auxiliary scanning direction along an axis of the rotatable drum to record the two-dimensional image on the recording sheet. With this arrangement, the image recorded on the recording sheet can maintain a desired level of dimensional accuracy irrespective of variations of the diameter of the drum.
The mechanical component may comprise a rotatable drum with the recording sheet mounted on an outer circumferential surface thereof, the image recording apparatus further comprising means for controlling the optical system to apply the light beam to scan the recording sheet on the rotatable drum in the main scanning direction to record the image on the recording sheet, and means for moving the optical system in the auxiliary scanning direction along an axis of the rotatable drum to record the two-dimensional image on the recording sheet.
The mechanical component may comprise a rotatable drum with the recording sheet mounted on an inner circumferential surface thereof, the image recording apparatus further comprising means for rotating the optical system about an axis of the drum to cause the light beam emitted from the optical system to scan the recording sheet on the rotatable drum in the main scanning direction to record the image on the recording sheet, and means for moving the optical system in the auxiliary scanning direction along the axis of the drum to record the two-dimensional image on the recording sheet.
The mechanical component correcting data may preferably be generated as main scanning component correcting data for correcting a graphical accuracy distortion in a circumferential direction of the drum, and auxiliary scanning component correcting data for correcting a graphical accuracy distortion in an axial direction of the drum.
According to the present invention, there is further provided an image recording apparatus comprising image recording means for scanning a recording sheet mounted on a mechanical component in a main scanning direction to record an image on the recording sheet per pulse of a pixel clock, the image recording means being movable in an auxiliary scanning direction substantially perpendicular to the main scanning direction to record a two-dimensional image on the recording sheet, means for detecting recording position information in the main scanning direction, original clock generating means for generating an original clock based on the recording position information in the main scanning direction, decimation counting means for counting pulses of the original clock and outputting a decimating instruction to decimate a pulse from the original clock each time a preset count is reached, decimating means for decimating a pulse from the original clock based on the decimating instruction, frequency-dividing means for frequency-dividing a decimated clock at a fixed frequency-dividing ratio and outputting the frequency-divided clock as a pixel clock for recording the image, storage means for storing mechanical component correcting data based on a relative positional relationship between the mechanical component and the image recording means, and decimating value calculating means for calculating the preset count from the mechanical component correcting data stored in the storage means and proportional component correcting data corresponding to recording conditions for the image recording means to record the image on the recording sheet, and setting the calculated preset count in the decimation counting means.
Since the decimating value calculating means calculates the preset count set in the decimation counting means from the mechanical component correcting data stored in the storage means and proportional component correcting data corresponding to recording conditions for the image recording means to record the image on the recording sheet, data for correcting the graphical accuracy can be generated and held efficiently with a resource saver against a distortion of an image, such as an expansion or a contraction, due to an error of a mechanical system for holding the recording sheet.
The image recording apparatus may further comprise means for detecting information per revolution of the drum, and random number generating means for generating a random number. The decimation counting means may comprise means for, when the count of the original clock is reset and thereafter the original clock starts to be counted to the preset count when the information per revolution of the drum is detected, setting a first preset count of the original clock after the count is reset to a value corresponding to the random number generated by the random number generating means, and outputting a decimating instruction to set a second and subsequent preset count of the original count to the preset count. Consequently, pixel clock positions are prevented from being corrected in fixed positions along the main scanning direction at all times, so that an image produced on the recording sheet does not suffer a quality degradation such as a striped irregularity or a moirxc3xa9 pattern.